Improve the documentation for new contributors

[antoyo]

Add a guide for new contributors. Topic ideas would include:

• Document the way to run a specific test (including the ones in tests/run).
• Document an existing function (explaining why it does things like casting values) to explain core concepts.
• Point to relevant documentation in the Rustc dev guide?
• Perhaps have a "Getting started" contribution guide to explain how a potential contributor can show their interest so that we can guide them (like join IRC/Zulip to ask us where to start)?

[g4titanx]

hey @antoyo what do you think about me being one of the maintainers for this project?

[antoyo]

@g4titanx: We could consider it when you have made more contributions.

[g4titanx]

alright, it's on then